1. Field of the Invention
The present invention relates to an electrical discharge tube, an illumination apparatus for a display device, a liquid crystal display device, and a liquid crystal display television.
2. Description of the Related Art
A liquid crystal panel for use with a liquid crystal display device such as a liquid crystal display television does not emit light by itself, and therefore needs an illumination apparatus for the display device separately as an external lamp, which is a so-called a backlight device. The backlight device is installed on the backside or on the side of the liquid crystal panel, and generally includes a metal base, a plurality of electrical discharge tubes, and a plurality of optical elements. The metal base has an open face on the liquid crystal panel side. The electrical discharge tubes are accommodated as lamps in the base. The optical elements (a diffuser plate and the like) are disposed over the open face of the base to efficiently pass light emitted by the electrical discharge tubes to the liquid crystal panel side.
Meanwhile, as the discharge tube used with such an illumination apparatus for the display device, a hot cathode fluorescent lamp is used in some cases. The hot cathode fluorescent lamp has emitter electrodes on both sides thereof. The emitter electrodes start discharging by being applied with discharging voltage therebetween while being heated. A lighting circuit for such a hot cathode fluorescent tube is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 5-242989. The publication discloses a device wherein, apart from a circuit for generating discharging voltage, a heating circuit that has a switching element and supplies current to emitter electrodes is provided. In the device, the switching element is intermittently switched for saving power.
With this type of a lighting circuit for the discharge tube, the longer the switched off state of the switching element continues, the more it contributes to the power saving. In some cases, however, the longer duration of the switched off state causes too much decrease in temperature in the emitter electrodes and decrease in the amount of the radiated thermoelectrons, and thus the discharge cannot be maintained. On the other hand, with longer duration of the switched on state of the switching element, it is impossible to save power. With longer duration of the switched on state, furthermore, the emitters are more wasted, and the life of the discharge tube is shortened.
Despite this, the switching control of the device disclosed in the above publication is operated based on a prediction of decrease in temperature after the power supply to the emitter electrodes is stopped. Therefore, with a construction wherein the actual temperatures in the emitter electrodes are affected by the current that flows through the emitter electrodes (that differs in every discharge tube) or external factors, it is difficult to keep the temperatures in the emitter electrodes in a most preferable state.